Metal louvers are installed to permit a free flow of ventilating air but to exclude rain, snow and other blown materials from the space to be ventilated. In most cases, therefore, all of the fins or vanes of a louver must be inclined in one and the same direction so that the louver will be effective to shed or deflect air-borne particles.
There is an increasing demand for relatively long metal louvers that are intended for installation in roof overhang soffits, having fins that extend lengthwise parallel to the length of the louver strip. Heretofore such soffit louvers have been produced by the stamping process, which required an expensive press and expensive dies and which was relatively slow because of the intermittent movement of the sheet metal as it was first advanced to feed it into the jaws of the press and then halted for closure of the press.
It was known, in general, that elongated articles such as soffit louvers could be produced more quickly and at lower cost by roller forming than by stamping. In roller forming the stock moves constantly at a substantially steady rate, in contrast to the stop-start movement of stock through a stamping press, and forming rollers are, as a rule, substantially less expensive than corresponding stamping dies because they can be made on a lathe.
Although the advantages of roller forming were obvious, soffit louvers have heretofore been regarded as inherently not adaptable to roller forming because of the previously unsolved problem of obtaining straightness of the finished product. When a strip of sheet metal is passed lengthwise through forming rollers, as for bending a flange along an edge of it or rolling a longitudinal ridge into it, the forming operation generates internal stresses in the metal that remain in it and tend to force it out of flatness. Such residual stresses are the result of lengthwise elongation or compression of the metal that occurred as the flange or ridge was forced out of the plane of the strip. If the strip has been roller formed to a configuration which is substantially symmetrical relative to its longitudinal centerline, the residual stresses left in it by the roller forming operation cause it to have essentially only a bow or curvature along its length, and the art is familiar with expedients for correcting this condition. However, in a strip of metal that has been roller formed to a louver with fins that extend parallel to its length and are all inclined in the same direction to the plane of the strip, the residual stresses resulting from the bending of the fins are asymmetrical to the longitudinal centerline of the strip and, moveover, such stresses reinforce one another all across the louvered width of the strip. As a result, the louver comes out of the fin forming rollers with a strong tendency to assume a marked corkscrew twist along its length.
In an effort to overcome this twisting tendency, various attempts were made at generating asymmetrical residual stresses in the louver that would offset or counterbalance the residual stresses resulting from the fin forming operation. Such attempts included the formation of a ridge in the material near one longitudinal edge of it that was subsequently rolled back to substantial flatness. None of these expedients was successful in completely and consistently eliminating twist, and some of them made matters worse by causing the louver to curve edgewise along its length.
Attempts to solve the twisting problem also showed that any truly satisfactory solution would have to be one whereby a change in the character or thickness of the metal being worked could be accommodated by the mere making of a simple adjustment, rather than requiring, for example, a modification of a forming roller or the making of a new forming roller on the basis of laborious calculations or tests.